Materials used for automotive brake disks are nowadays predominantly made of steel or cast iron, and in aircraft applications of carbon materials reinforced with carbon fibers (C/C). The properties required of the disk materials are high mechanical stability, heat resistance, hardness and wear resistance in the friction pair of a brake. The use temperature of cast iron brake disks used hitherto is limited by the melting point of the material. The temperature at which mechanical failure occurs is, depending on the stress, significantly below the melting point. Furthermore, there is a risk of cracking of the disks due to transformation of the metallic microstructure on heating. The use of fiber-reinforced ceramics as a material for brake disk applications has been found to be a solution to these problems. Materials based on silicon carbide reinforced with carbon fibers (C/SiC) in particular have been found useful for this application. The advantages of this material are their lower density (reduced weight for a given volume), their high hardness and heat resistance up to about 1400° C. and, not least, their extremely high wear resistance. The significantly reduced density of brake disks made of these C/SiC materials improves comfort and safety by reduction of the unsprung masses in motor vehicles, and also economics in aircraft applications. The high hardness and wear resistance of C/SiC components allows to achieve far longer operating lives compared to previously customary materials based on C/C or metal.
Processes for producing C/SiC components have been known from, for example, DE-A 198 56 721, DE-C 197 11 829 and DE-A 197 10 105 and comprise, inter alia, the following steps:                preparation of a pressable mixture or formable composition comprising, firstly, carbon-containing fibers or fiber bundles which may be coated and, secondly, fillers and/or binders such as resins and/or pitch,        shaping of the mixture under pressure and at elevated temperature and carbonization of the carbon-containing fillers and binders to produce a shaped body, in particular a shaped body comprising carbon reinforced with carbon fibers (C/C) and, if appropriate, subsequent graphitization,        infiltration of at least an outer layer of the shaped body with a silicon melt and at least partial reaction with the carbon in the shaped body to produce SiC, thus forming a shaped body which comprises, at least in the outer layer, a composite ceramic composed of carbon-containing fibers embedded in a matrix comprising predominantly SiC, Si and C (hereinafter referred to as C/SiC).        
In the following, the term C/SiC materials also encompasses the material variant in which, as described above, only an outer layer is silicized.
The term “formable fiber composition” encompasses both the fiber-containing press moulding compositions which typically comprise short fibers or short fiber bundles and also fiber mats, woven fabrics or nonwovens which can be processed, for example, by the prepreg technique. The latter can also, in particular, be shaped with very little or no application of pressure.
Customary production processes also include those in which the C/C body is post-densified via the liquid or gas phase with carbon precursors, namely substances which form carbon on heating in the absence of oxidizing media, or by means of carbon, or the matrix comprising predominantly SiC, Si and C is produced by gas-phase infiltration (CVD, chemical vapor deposition, or CVI, chemical vapor infiltration) or by pyrolysis of Si— containing preceramic polymers.
During pressing of the green body, the heat necessary for thermal curing of the binder is generally introduced from the outside into the moulding composition or the workpiece by heating the press or at least the punch of the press. As a result, the outer region of the workpiece is heated to a temperature higher than that of the inner region, so that heat can be transported into the interior of the workpiece by means of the temperature gradient. This nonuniform heating leads to nonuniform curing and can lead to stresses in the workpiece; the chemical and physical processes occurring in the outer zone can even lead to liberation of gases, for example, during heating and the accompanying chemical reactions which cannot escape through the outer zone and may lead to rupture of the workpiece or to formation of cracks in the workpiece. In addition, the total time of the pressing operation, viz. the pressing cycle time, is undesirably high, since all press components which are in contact with the press moulding composition, which sometimes have a considerable heat capacity, have to be heated and cooled again. This causes a high energy consumption.
It is therefore an object of the invention to provide a process, in particular a process having short cycle times and a low energy consumption, which is suitable for producing fiber-reinforced carbon-containing green bodies or intermediate bodies which may, if desired, subsequently be converted into shaped bodies comprising fiber-reinforced carbide ceramic by infiltration with liquid metals, in particular liquid silicon, and subsequent reaction.